Method for utilizing gas reserves with low methane concentrations and high inert gas concentrations for fueling gas turbines

ABSTRACT

The invention is directed to a method of fueling gas turbines from natural gas reserves with relatively low methane concentrations. The invention uses such reserves to generate electric power. The invention permits the use of these reserves at significantly lower cost than by producing pipeline natural gas to fuel gas turbines to generate electric power. These reserves currently generally are used only after the removal of impurities to produce pipeline natural gas quality turbine fuel. The latter current technology is capital intensive, and at current natural gas prices, economically unattractive. The process of the invention can remove the impurities from the gas from the natural gas reserve necessary for protection of the environment and leaves inert gasses in the fuel in an amount which will increase the output of a gas turbine for the generation of power by about 5 to about 20%.

This invention relates to the combustion of natural gas havingrelatively low methane concentrations and relatively high concentrationsof inert gases. More particularly, this invention relates to theutilization of natural gas reserves having methane gas concentrations offrom above 40 to about 80 volume percent methane by increasing therelative concentration of inert gases and using the methane producedthereby in a process which produces pipeline natural gas. In one aspect,blending hydrogen gas to provide an inert gas and hydrogen enhancedmethane gas blend with a methane gas concentration of not more thanabout 40 volume percent, based upon the total volume of the gas. Thisgas blend is used for fuel in gas turbines.

BACKGROUND OF THE INVENTION

Currently there are substantial methane gas reserves with relatively lowmethane gas concentrations. Many of these reserves have methane gasconcentrations from about 40 to about 80 volume percent. Currently,impurities are removed from natural gas to make pipeline quality naturalgas which normally have methane concentrations of from about 95+ toabout 99+volume percent. To fuel gas turbines to make electric power,converting natural gas having methane concentrations of from about 40 toabout 80 volume percent methane to pipeline quality natural gas becomeseconomically impractical because the conversion is capital intensive.Moreover, natural gas with methane concentrations in the range of 40 to80 volume percent does not necessarily provide a reliable fuel sourcefor gas turbines, especially at the low end of the latter range, togenerate power with enhanced outputs of power because natural gas withsuch low methane concentrations will not provide a stable flame for fuelcombustion without special equipment designs, catalysts and withoutspecial balancing of oxygen with other combustibles. Moreover, streamswith 40 to 80 volume percent methane have the problem of NOx as a resultof higher flame temperatures in the turbine.

It would be economically advantageous to utilize natural gas reserveswith 40 to 80 volume percent methane and with large amounts of inertgases and purify such streams or a portion of such streams to provide afuel for gas turbines at significantly lower cost than by producingpipeline quality natural gas for fuel for gas turbines.

SUMMARY OF THE INVENTION

The invention is directed to a method of fueling gas turbines fromnatural gas reserves with relatively low methane concentrations of fromabout 40 to about 80 volume percent and relatively high inert gasconcentrations. The invention permits the use of these reserves atsignificantly lower cost than by producing pipeline natural gas to fuelgas turbines to generate electric power. As described, these reservescurrently generally are used only after the removal of impurities toproduce pipeline natural gas quality turbine fuel. Also as previouslydescribed, the latter current technology is capital intensive, and atcurrent natural gas prices, economically unattractive. The process ofthe invention can remove the impurities from the gas from the naturalgas reserve necessary for protection of the environment, and leavesinert gases in the fuel in an amount which will increase the output of agas turbine for the generation of power by about 5 to about 20%. In oneaspect the process of the invention contemplates leaving the inert gasesin the fuel to maximize mass flow through the gas turbine and toincrease power output without the expense of producing pipeline qualitymethane gas and blending additional inert gases into the fuel toadditionally enhance the mass flow through the turbine and lower flametemperature to reduce Nox.

In one aspect the process of the invention uses a natural gas stream andprocess which is being used to produce pipeline quality natural gas. Inthis aspect during the purification process inert gases are separatedfrom methane gas in the natural gas reserves with from about 40 to about80 volume percent methane. A separation may be made which removes onlysuch an amount of methane to provide an inert enhanced methane gas blendwhich is effective for providing an increase in output of power by a gasturbine of by about 5 to about 20% as compared to a turbine fueled withpipeline quality natural gas. The purified methane stream generated bythis separation then may be sent for further purification to producepipeline quality natural gas. Alternatively here may be a generalseparation of inert gases from the methane gas and the inert gasesseparated from the methane then are mixed with the natural gas streamwhich has not had the inert gases removed (or another gas stream with 40to 80 volume percent methane) in an amount which will increase theoutput of the gas turbine by about 5 to about 20% as compared to aturbine fueled with pipeline quality natural gas.

In one aspect where nitrogen gas is the primary inert gas, membranes areused to do the primary separation of the inert nitrogen gas from thenatural gas and methane therein. Membranes commercially available andsold under the name of Medal from Air Liquide, Houston Tex., aresuitable for such separation. This separation may be done only in anamount to increase the output of a turbine as previously described, orthe nitrogen separated from the natural gas then is combined with thenatural gas reserve to provide and inert enhanced natural gas which willincrease the output of the turbine by about 5 to about 20% as comparedto a turbine fueled with pipeline quality natural gas.

Membranes may be used to separate carbon dioxide and methane when thecarbon dioxide concentration is up to about 45 volume percent. Inanother aspect where carbon dioxide is the primary inert gas atconcentrations above about 45 volume percent, the carbon dioxide isseparated from the natural gas reserve containing from about 40 to about80 volume percent methane cryogenically. In one aspect when the pressureof the natural gas is high, such as greater then about 2500 psig, thehigh pressure feed gas is flashed to a lower pressure, such as about 500psig. A Joule Thomson effect on this expansion is in an amount effectiveto provide a cooling to do a cryogenic separation. When the pressure ofthe natural gas reserve is low, such as below about 1100 psig theseparation may be achieved an external refrigeration of the natural gasreserve to provide the cooling effective for separating the carbondioxide from the natural gas reserve and the methane there.

In a very important aspect, inert gases are separated from the naturalgas reserve and the methane therein to provide a methane gas with lessthan about 40 volume percent methane. The methane separated from thenatural gas reserve then may be sent for further processing to makepipeline equality natural gas. The inert gases separated from thenatural gas stream then are mixed back into the natural gas stream fromthe well in an amount effective for providing a methane/inert gas blendhaving less than about 40 volume percent methane to provide an inertenhanced methane gas blend. The inert enhanced methane gas blend isblended with hydrogen, or in an important aspect, just enough methane isshifted to hydrogen gas to produce a gas fuel blend of hydrogen/inertgas/methane gas that not only is an acceptable fuel for gas turbines,but the blend is effective for providing flame stability (such asproviding the gas with at least 110 BTUs per standard cubic foot of gas)and for producing more power than a standard natural gas having fromabout 95+ to about 99+volume percent methane. If required, the hydrogeninert gas/methane gas blend is dehydrated to remove a sufficient amountof water to provide a flame stable dehydrated hydrogen/inert gas/methanegas blend. In an important aspect, this blend has at least about 6volume percent hydrogen. The flame stable hydrogen enhancedhydrogen/inert gas/methane gas blend then is used to fuel an electricpower producing gas turbine. In this aspect, the method of the inventionis effective for increasing the power output of a gas turbine by atleast about 10 percent as compared to gas turbine using a standardnatural gas which comprises from about 95 to about 99+volume percentmethane. In most cases in this aspect, output may be increased by atleast about 20 and up to about 30 percent with these latter limits beingimposed by the mechanical design limitations of the gas turbine.

In the aspect of the invention where methane is shifted to hydrogen gas,a portion of the methane from natural gas is catalytically converted orreformed into hydrogen gas which then forms the hydrogen enhancedmethane/hydrogen gas blend. The reactions which to obtain thisconversion include:CH₄+CO₂→2CO+2H₂CH₄+2H₂O→CO₂+4H₂

In an another important aspect, especially where methane is shifted tohydrogen, prior to the conversion reaction, hydrogen sulfide and otheracid components such as COS, RSH and RSSR are removed from the naturalgas using a physical solvent to provide a sweet natural gas. A physicalsolvent selectively removes hydrogen sulfide and other acid gases, butminimizes the removal of inert gases, such as nitrogen and carbondioxide and other inert gases such as helium and aragon. In this aspect,the physical solvent is selected from the group consisting of methanol,a blend of dimethyl ethers of polyethylene glycol (molecular weightabout 280), propylene carbonate (b.p.of240° C., N-methyl-2-pryrrolidone(b.p. 202° C.), a blend of oligoethlene glycol methyl isopropyl ethers(b.p. about 320° C.), tri-n-butyl phosphonate (b.p. 180° C. at 30 mm Hg)and methyl cyano-acetate (b.p. 202° C.). The sweet natural gas is mixedwith sufficient water to permit sufficient production of hydrogen fromthe methane to achieve flame stability or a BTU/Scf value of at leastabout 110. In this aspect, it is important that the hydrogen sulfide andother acid gases are removed prior to reforming a portion of the methaneto hydrogen because the reformation is a catalyzed reaction where thecatalyst may be poisoned by the hydrogen sulfide gas and other acidgases. Catalysts which are sensitive to the acid conditions and whichmay be used in this aspect of the invention include the C11 Seriescatalyst from United Catalyst Inc., R67 from Haldor Topsoe and G1-25from BASF. High temperature “shift catalysts” for sweet natural gasgenerally are made from copper, zinc and aluminum.

In another important aspect, the reformation reaction is done under acidor sour conditions using catalysts such as a C25 Series catalyst fromUnited Catalyst Inc., K8-11 catalyst from BASF and SSK catalyst fromHaldor Topsoe. In general these catalysts are chrome/molybdenumcatalysts. In this aspect of the invention, the sour natural gas andwater are mixed with the water being in amount which will result in amethane gas/water blend which will permit the formation or reformationof sufficient hydrogen gas to provide flame stability for the hydrogenenhanced inert gas/methane gas/hydrogen gas blend which does not have inexcess of 40 volume percent methane.

After blending the inert gases with the natural gas reserves, the inertenhanced methane gas blend may have as low as 35, 25 or even less than20 volume percent methane and still provide more power than pipelinequality methane when using hydrogen to provide flame stability toprovide a fuel for gas turbines. In practicing this aspect of theinvention, sufficient methane should be converted into hydrogen toproduce a hydrogen enhanced hydrogen/inert gas/methane gas blend with atleast 6 volume percent, and preferably from about 6 to about 10 volumepercent hydrogen. This will produce a hydrogen enhanced hydrogen/inertgas/methane gas blend with stable flammability that is very effectivefor fueling gas turbines for the generation of electric power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow plan illustrating the process of the invention wheresome of the methane is shifted to hydrogen gas to make a hydrogen/inertgas/methane gas blend for fuel for a gas turbine.

FIG. 2 is a flow plan illustrating the process of the invention wherehigh pressure feed natural gas is flashed to a lower pressure toseparate methane gas from carbon dioxide gas both of which are presentin the high pressure feed natural gas.

FIG. 3 is a flow plan illustrating the process of the invention wherelow pressure feed natural gas is refrigerated using an externalrefrigeration device to separate methane gas from carbon dioxide gasboth of which are present in a low pressure feed natural gas.

DETAILED DESCRIPTION OF THE INVENTION

A natural gas reserve having from about 40 to about 80 volume percentmethane and a high relative concentration of inert gas such as nitrogen,carbon dioxide, helium and aragon is conveyed to a separation unit toseparate the inert gases from the methane gas in the natural gasreserve. This produces a relatively pure methane stream and an inertgas/methane stream. If the reserve has high nitrogen content, thenitrogen gas can be separated from the methane gas using membranes, suchas the Medal membranes. If the inert gas is carbon dioxide, the inertgas may be separated from the methane gas using a cryogenic separationas seen in FIGS. 2 and 3. After the methane gas is separated from theinert gas, the separated relatively pure methane gas may be conveyed tofurther purification apparatuses as are known to produce pipelinequality natural gas. If after the separation of the inert gas from themethane gas the inert gas/methane stream does not have sufficient inertgas to increase the power output of a gas turbine as described, thenfurther inert gases may be added to the inert gas methane stream toincrease the power output by from about 5 to about 20% as compared to aturbine fueled with pipeline quality natural gas. Alternatively, aportion of the natural gas stream which has not been separated from itsinert gas components is further blended with inert gases, which havebeen separated from the reserve gas stream in an amount to provide aninert enhanced natural gas which will increase the output of the turbineby about 5 to about 20% as compared to a turbine fueled with pipelinequality natural gas.

In one important aspect, sufficient inert gases may be mixed with theinert gas/methane stream or with the natural gas reserve to lower themethane content of the methane gas in such a blend to less than 40volume percent. In this aspect as can be seem by reference to FIG. 1 theinert enhanced methane gas blend having a methane concentration of notmore than about 40 volume percent is moved treated with a physicalsolvent such as methanol, a blend of dimethyl ethers of polyethyleneglycol, propylene carbonate, N-methyl-2-pryrrolidone, a blend ofoligoethlene glycol methyl isopropyl ethers, tri-n-butyl phosphonate,and methyl cyano-acetate to remove hydrogen sulfide gas and other acidgas components without removing inert gases to provide a sweet naturalgas with not more than about 40 volume percent methane. Treatment withthe physical solvent can be before or after separating the inert gasfrom the methane gas in the natural gas feed. The sweet inert methanegas blend then is conveyed via line 1 to a zinc oxide guard bed 2 toprevent the emissions of hydrogen sulfide gas. The sweet inert methanegas blend is conveyed from the zinc oxide bed and mixed with water inline 3 to provide an inert enhanced methane gas/water blend. Thegas/water blend is conveyed at about 70° F. and at about 355 psig inline 3 to a feed effluent heat exchange 4 where the temperature of thesweet inert enhanced methane gas/water blend is raised to about 800° F.Sufficient water to be mixed with the gas blend to permit sufficientconversion to hydrogen to provide flame stability to a dehydratedhydrogen enhanced inert gas/methane/hydrogen gas blend when it isdelivered to the gas turbine generator for the generation of power.After the sweet inert enhanced methane gas blend is mixed with water andheated in the feed effluent exchanger, the heated sweet inert enhancedmethane gas/water blend is conveyed via line 5 at about 345 psig andabout 800° F. to a heat recovery steam generator coil (HRSG coil) tofurther raise the temperature of the sweet inert enhanced methanegas/water blend and provide a hot sweet gas/water blend having atemperature of about 950° F. in line 3. The hot sweet gas/water blendthen is conveyed via line 7 to a reforming reaction chamber 8 at about340 psig for converting a part of the methane in the sweet inertenhanced methane gas/water blend to a hydrogen enhanced inertgas/methane/hydrogen gas/water blend. The methane in the sweet gas/waterblend undergoes a catalyzed reaction to react the methane and water toproduce hydrogen gas at least about 700° F. and preferably from about900 to about 950° F. and about 340 psig. Higher temperatures facilitatethe conversion, while higher pressures adversely affect the conversion.Pressure should not exceed 1500 psig. After conversion of sufficientmethane to hydrogen to provide at least about 6 volume percent hydrogenin the gas-present after dehydration (hereinafter described), thehydrogen enhanced inert gas/methane/hydrogen gas/water blend is conveyedback to the feed effluent exchanger via line 9 at about 855° F. and 335psig to transfer heat to the water and methane gas entering the feedeffluent exchanger. After the temperature of the hydrogen enhanced inertgas/methane/hydrogen gas/water blend is reduced, it is conveyed via line10 to a dehydrating knockout drum (KO drum) 12 to reduce the watercontent of the hydrogen enhanced inert gas/methane/hydrogen gas blend.The dew point is lowered in the KO drum to permit water to condense andseparate from the gas. Sufficient water is removed to permit flamestability and provide the gas with at least about 110 BTIUs per standardcubic foot of gas. In general, from about 97 to about 99 or more weightpercent of the water is removed from the gas. The water resulting fromdehydrating the hydrogen enhanced inert gas/methane/hydrogen gas/waterblend is removed from the KO drum 12 via line 14 using condensate pump16 and is conveyed back to the feed effluent exchanger 4 via line 18 atabout 100° F. at about 500 psig. The dehydrated hydrogen enhanced inertgas/methane gas/hydrogen gas blend which now has at least 6 volumepercent hydrogen or sufficient hydrogen gas to provide flame stabilityis fed from the KO drum to a gas turbine generator via line 20 at about100° F. at about psig. There the gas has at least about 110 BTUs perstandard cubic foot of gas and provides a stable flame from the gasturbine generator. The same process may be used in utilizing a sournatural gas using a catalyst which will not be sensitive or poisoned bythe acid gases in the natural gas. To keep the process compatible withthe environment, however, at least some of the acid gases such as H₂smay be removed at least in part.

The invention is illustrated by the following example.

EXAMPLE I

GAS TURBINE PERFORMANCE Units No Aug Power Aug Site Conditions AmbientTemperature ° F. 60 60 Ambient Pressure psia 11.57 11.57 AmbientRelative % 60 60 Humidity Inlet Pressure Drop In H₂O 3.0 3.0 PerformanceGross Generator kW 156,100 157,100 Output Heat Consumption Btu/h × 10⁻⁸1,477.1 1,486.9 (LHV) Heat Rate (LHV) Btu/kWh 9,461 9,462 Misc. 1.8 1.8LHV Btu/Lb 2,424.0 2,424.0 Btu/Scf 212.5 212.5 Fuel Gas Flowrate lb/s169.3 170.4 Pressure psia 325 325 Temperature ° F. 80 80 Power Aug InjConditions Composition % Vol Carbon Dioxide 100.0 100.0 Flowrate lb/s0.0 4.0 Pressure psia 285 285 Temperature ° F. 300 300 Exhaust GasConditions Exhaust Gas Flow lb/s 925.3 930.4 Exhaust Gas ° F. 1,093.11,095.1 Temperature Exhaust Gas % Vol Composition Carbon Dioxide 13.7914.12 Argon 1.07 1.07 Nitrogen 65.78 65.53 Oxygen 9.83 9.74 Water 9.539.55 Exhaust Pressure Drop In H₂O 15.0 15.0 Nox (Thermal) ppmvd @ 15% O₂<10 <10

Data with respect to FIG. 2 is attached hereto as Appendix for FIG. 2,pages 1-3. Data with respect to FIG. 3 is attached hereto as Appendixfor FIG. 3, pages 1-3.

1-26. (canceled)
 27. A method of fueling a gas turbine with methane gasfrom natural gas reserves having from about 40 to about 80 volumepercent methane, the method comprising: providing a natural gas streamfrom the natural gas reserve; separating inert gases from methane gas inthe natural gas stream to provide a separated methane gas and aseparated inert gas/methane gas blend wherein the separation is madesuch that when the inert gas methane gas blend is used as turbine fuelthere is an amount of inert gases in the blend which is effective forpermitting the blend to increase an output of the gas turbine by atleast about 5 percent as compared to a turbine fueled with pipelinequality natural gas. 28-30. (canceled)
 31. A method of fueling a gasturbine with methane gas from natural gas reserves having from about 40to about 80 volume percent methane, the method comprising; providing afirst natural gas stream from the natural gas reserve; separating inertgases from methane gas in the first natural gas stream to provide aseparated methane gas and a separated inert gas/methane gas blend havingless than about 40 volume percent methane; removing at least one acidcomponent from the separated inert gas/methane gas blend to provide asweet inert enhanced inert gas/methane gas blend; mixing the sweet inertenhanced inert gas/methane gas blend and water to provide a hydratedsweet inert enhanced inert gas/methane gas blend, the water in thehydrated sweet inert enhanced inert gas/methane gas blend being in anamount effective for permitting the conversion of a portion of themethane in the sweet inert enhanced inert gas/methane gas blend tohydrogen gas and effective for providing a flame stable dehydratedhydrogen enhanced inert gas/methane gas/hydrogen gas blend.Catalytically converting a portion of the methane to hydrogen gas in thehydrated sweet inert enhanced inert gas/methane gas blend to provide ahydrogen enhanced inert gas/methane gas/hydrogen gas blend, theconversion effective for providing the flame stable dehydrated hydrogenenhanced inert gas/methane gas/hydrogen gas blend; dehydrating hydrogenenhanced inert gas/methane gas/hydrogen gas blend to provide flamestable dehydrated hydrogen enhanced inert gas/methane gas/hydrogen gasblend; and fueling a gas turbine with the dehydrated flame stabledehydrated hydrogen enhanced inert gas/methane gas/hydrogen gas blend.32. The method for fueling a gas turbine as recited in claim 31 whereinthe methane in the hydrated sweet inert enhanced inert gas/methane gasblend is catalytically converted using a shift catalyst selected fromthe group consisting of iron/chrome/copper, copper/zinc/aluminum andmixtures thereof.
 33. The method as recited in claim 32 wherein thesweet inert enhanced inert gas/methane gas blend does not have more thanabout 35 volume percent methane and the dehydrated hydrogen enhancedinert gas/methane gas/hydrogen gas blend comprises from about 6 to about10 volume percent hydrogen gas.
 34. (canceled)
 35. A method of fueling agas turbine with methane gas from natural gas reserves having from about40 to about 80 volume percent methane, the method comprising; providinga first natural gas stream from the natural gas reserve; separatinginert gases from methane gas in the first natural gas stream to providea separated methane gas and a separated inert enhanced inert gas/methanegas blend having less than about 40 volume percent methane; mixing theseparated inert enhanced inert gas/methane gas blend and water toprovide a hydrated inert enhance inert gas/methane gas blend, the waterin the hydrated sweet inert enhanced inert gas/methane gas blend beingin an amount effective for permitting the conversion of a portion of themethane in the inert enhance inert gas/methane gas blend to hydrogen gasand effective for providing a flame stable dehydrated hydrogen enhancedinert gas/methane gas hydrogen gas blend; Catalytically converting aportion of the methane to hydrogen in the inert enhanced inertgas/methane gas/water blend to provide a hydrated hydrogen enhancedinert gas/methane gas/hydrogen gas blend, the conversion effective forproviding the flame stable dehydrated hydrogen enhanced inertgas/methane gas/hydrogen gas blend; dehydrating hydrated hydrogenenhanced inert gas/methane gas/hydrogen gas blend to provide flamestable dehydrated inert enhanced inert gas/methane gas/hydrogen gasblend; and fueling a gas turbine with the flame stable dehydratedhydrogen enhanced inert gas/methane gas/hydrogen gas blend.
 36. Themethod for fueling a gas turbine as recited in claim 35 wherein themethane in the inert enhanced inert gas/methane gas/water blend iscatalytically converted using a chrome/molybdenum catalyst.
 37. Themethod as recited in claim 36 wherein the inert enhanced inertgas/methane gas blend does not have more than about 35 volume percentmethane and the dehydrated hydrogen enhanced inert gas/methanegas/hydrogen gas blend comprises from about 6 to about 10 volume percenthydrogen gas.
 38. (canceled)